The typical modern wind turbine is a dynamic system having many moving parts that facilitate converting the kinetic energy of the wind into electrical energy. In this regard, a wind turbine generally includes a cantilevered tower having a lower end rigidly secured to a base or foundation and an upper end that is free or unsupported, a nacelle located adjacent the upper end of the tower and housing a generator capable of converting mechanical energy into electrical energy, and a rotor having a central hub and a plurality of blades supported by the nacelle and capable of converting the kinetic energy of the wind into mechanical energy (e.g., rotation of a shaft). The rotor is operatively coupled to the generator housed inside the nacelle such that when wind of sufficient speed moves across the blades, the rotor rotates to thereby power the generator to produce electrical energy.
Like most dynamic systems, wind turbines are subject to undesirable vibrations that may detrimentally impact the operation and/or structural integrity of the wind turbine. Additionally, vibrations in wind turbines may be exacerbated since the forcing function (e.g., the wind) acting on the structural elements of the wind turbine may be spatially non-uniform and unsteady in time. In any event, these undesirable vibrations often present themselves as bending and torsional vibrations within the wind turbine tower. Moreover, these bending and torsional vibrations may have resonance values (e.g., large amplitude oscillations at a specific frequency) within the operating range of the wind turbine. Accordingly, to minimize damage to the wind turbine, the design should account for these undesirable vibrations.
One design approach is to structurally reinforce the wind turbine so as to alter its vibration response (e.g., make the tower stiffer). Such a solution, however, may be prohibitively expensive, especially as tower heights exceed 60-70 meters and will most likely increase in future designs. Another design approach is to not stiffen the structural elements of the wind turbine, but to allow the vibrations and specifically address their impact through supplemental systems. In this regard, various vibration dampers have been suggested that reduce or minimize the effects of resonant vibrations in wind turbines. These dampers may, for example, reduce the large-amplitude oscillations characteristic of resonant behavior. In one form or another, however, these vibration dampers have certain drawbacks that do not fully address the potential negative impact of resonant vibrations on the wind turbine.
By way of example, vibration dampers have been implemented that are specifically configured to minimize the effects of the resonant frequency in a first bending direction, e.g., movement of the tower back and forth in a direction generally parallel to the wind direction, as illustrated by arrow B1 in FIG. 1. While being generally effective for minimizing the impact of this type of bending vibration on the wind turbine, such vibration dampers fail to adequately address the torsional vibrations acting on the wind turbine, and the wind turbine tower in particular. Thus, damage to the wind turbine may still occur through this unaddressed vibratory mode.
Other vibration dampers have been developed that purportedly address both bending and torsional vibrations. These so-called hybrid vibration dampers, however, also have certain drawbacks which may leave the wind turbine vulnerable to vibration-induced damage. More particularly, hybrid vibration dampers can be difficult to “tune” to the resonant frequency of both bending and torsional vibrations, especially when the respective resonant frequencies differ by a significant amount. In this regard, in many cases, the resonant frequency in the first bending direction is not the same as the resonant frequency in a first torsional direction, e.g., twisting about an axis extending along the length of the tower, as illustrated by arrow T1 in FIG. 1. Thus, when the hybrid vibration damper is tuned to the resonant frequency in the first bending direction, which is typical in current hybrid vibration dampers, the damper is not optimally tuned to the resonant frequency in the first torsional direction. Accordingly, the impact of the torsional vibration on the wind turbine may not be adequately addressed. Thus, while on the one hand the coupled nature of hybrid vibration dampers provides dampening in multiple directions (e.g., in a bending and torsional direction), on the other hand the ability to specifically target the resonant frequency in both directions is compromised. Accordingly, prophylactic measures to minimize damage from undesirable vibrations, and more particularly from resonant torsional vibrations, are not being fully implemented through current vibration damper designs.
Thus, there is a need for a vibration damper that decouples the bending vibration from the torsional vibration and is specifically designed to minimize the effects of torsional vibrations on the wind turbine at its resonant frequency.